Like Saturn's moon Enceladus, Jupiter's moon Europa has a subsurface ocean that might support earthly life, according to the new Standard Microbial Habitability index, announced in October 2009.

The purported habitable zone on Europa--which earned a 0.3 on the index--is not as large as that of Saturn's Enceladus, but the ocean is shallower and perhaps easier to explore than that of Enceladus, said Mendez, of the University of Puerto Rico at Arecibo.

But even if a world has potentially habitable environments, that doesn't guarantee life is present, Mendez cautioned.

"Life is a complex process," he said.

"Even if life started or was introduced in any of these bodies, it requires a relatively stable environment to maintain it."